1. Field of the Invention
The present invention generally relates to optical communication arts, and more particularly to an optical element that serves for a high-capacity optical communication and achieves stable performance with a simple structure.
2. Description of the Related Art
In recent high-capacity optical communication arts, WDM (Wavelength Division Multiplexing) is typically used to deal with large traffic volume. In the WDM, optical signal components each of which has a different wavelength from each other are formed in correspondence with a large number of signals, and these optical signal components are multiplexed to form a single wavelength multiplexed optical signal.
On the other hand, in order to further increase a communication capacity in the WDM, OTDM (Optical Time Division Multiplexing) or TWDM (Time Wavelength Division Multiplexing) are presented as an optical communication technique for performing a time division multiplexing for individual wavelength components.
While the WDM is designed to improve the signal density by wavelength-multiplexing a signal light, the optical time division multiplexing technique such as the OTDM and the TWDM aims at further improving the signal density of a pulse light that has an extremely narrow spectrum width with respect to wavelengths of individual components and ultimately achieving the transmission rate higher than 160 Gbit/s.
In implementation of an optical signal transmission under the OTDM by using a conventional technique, it is necessary to temporarily convert an optical signal entering at the transmission rate higher than 160 Gbit/s into an electric signal through a photoelectric conversion so as to perform timing extraction, waveform reshaping and signal regeneration and then convert the resulting electric signal into an optical signal through an electricphoto conversion. In this case, since the response speed of the electric signal is restricted by the traveling time of a carrier in a photo diode used for the photoelectric conversion, it is impossible to achieve the response speed required to detect the optical signal even if a high-speed PIN photo diode is used.
At present, the limit of the detection speed of an electric signal is about 40 Gbit/s. In order to process an OTDM signal having the speed higher than 40 Gbit/s, it is necessary to divide an optical signal by performing a high-speed optical signal process and then perform an optical asymmetric demultiplexing process for the divided optical signals so as to fall the bit rate at the feasible speed for electric processes.
In response to the above-mentioned circumstance, there have been some researches on devices to process an optical signal without converting into an electric signal. An optical asymmetric demultiplexing switch for controlling an optical signal and an all-optical signal waveform reshaping element for directly reshaping an optical signal waveform without conversion are typical as the devices to process an optical signal without converting into an electric signal.
A description will now be given, with reference to accompanying drawings, of the above-mentioned conventional techniques and devices.
FIG. 1 shows a structure of an optical asymmetric demultiplexing signal processing system 10 that is constructed according to a currently available technique.
The timing extraction is a primary technique for the optical asymmetric demultiplexing process. The timing extraction serves to synchronize a process with a signal light. In FIG. 1, the timing extraction is performed by partially using an electric element.
In FIG. 1, a beam splitter 11 splits an input signal light into a first branch 11A and a second branch 11B. The signal light in the first branch 11A is directly delivered to an optical switch 12. In contrast, the signal light in the second branch 11B is photoelectric-converted by a photo diode 13. The converted electric signal passes through a PLL (Phase Locked Loop) circuit 14 so that the PLL circuit 14 can be synchronized with a synchronous signal of the converted electric signal. Then, an output of a timing generator 15A synchronizing with the PLL circuit 14 oscillates a pulse laser 15 for generating a control light. The generated control light is supplied to the optical switch 12, and the optical switch 12 switches the signal light in the first branch 11A. As a result, it is possible to achieve a desired DEMUX.
However, the above method is not preferred with respect to the fabrication cost in that the optical asymmetric demultiplexing signal processing system 10 has the complicated and large structure because the optical asymmetric demultiplexing signal processing system 10 includes the conversion process to temporarily convert the input optical signal into the electric signal. Additionally, it is difficult to implement a light receiving element for detecting the high-speed optical signal traveling at 160 Gbit/s under such a structure.
FIG. 2 shows a structure of a conventional optical asymmetric demultiplexing signal processing system 10A in which a mode-locked laser is used to eliminate the above-mentioned problem, wherein those parts corresponding to the parts in FIG. 1 are referred to as the same reference numerals and the description thereof will be omitted.
In FIG. 2, while the beam splitter 11 splits a signal light into the first branch 11A and the second branch 11B like the method described with respect to FIG. 1, a mode-locked laser 16 is provided to directly receive the signal light in the branch 11B with no conversion. Since a mode lock is generated in the mode-locked laser 16 by involving in timing of the signal light, it is possible to obtain a pulse oscillation of the required timing. Consequently, when an output light of the mode-locked laser 16, which serves for the mode-locked oscillation, is delivered to the optical switch 12, it is possible to achieve a desired DEMUX.
In the structure in FIG. 2, it is possible to deal with all processes without any conversion of the optical signal into an electric signal because there is no photoelectric conversion and no process on which an electric circuit is used. As a result, it is possible to simplify the structure of the optical asymmetric demultiplexing signal processing system 10A.
However, it is also necessary for the beam splitter 11 to split the signal light according to the structure in FIG. 2. Furthermore, it is necessary to combine the optical switch 12 and the mode-locked laser 16. Obviously, it is difficult to integrate the two parts in practice.
A description will now be given, with reference to FIGS. 3A through 3C, FIG. 4 and FIG. 5, of a conventional technique regarding the optical switch 12 and the mode-locked laser 16 and some objects of the conventional technique in conjunction with the above-mentioned problem.
Conventionally, various all-optical interference devices are presented for the optical switch 12: a NOLM (Nonlinear Optical Loop Mirror) optical switch 20 in FIG. 3A, a Mach-Zehnder optical switch 30 in FIG. 3B and a polarization-discriminating optical switch 40 in FIG. 3C.
In FIG. 3A, an optical fiber loop 21 is formed of an optical fiber 21A in the NOLM optical switch 20. An injected signal light is divided into optical signal components, and the optical signal components are transmitted in the optical fiber loop 21 in such a way that one of the optical signal components can travel in the opposite direction to the other optical signal component. In the optical fiber loop 21, a waveguide 21B (hereinafter referred to as a nonlinear waveguide) is provided at an asymmetric position to the position in which the signal light is injected. The nonlinear waveguide 21B changes a refractive index in accordance with a control light from an exterior. Then, when the traveling direction of the signal light components are reversed in the optical fiber loop 21, there arises a differential phase shift by the timing gap when the signal light components pass through the nonlinear waveguide 21B. As a result, when the two signal light components are multiplexed, the phase difference switches the signal light.
As mentioned above, the NOLM optical switch 20 can form the all-optical switch 12 in a comparatively simple structure. However, in order to operate the NOLM optical switch 20, it is necessary to pass a first pulse through a gain medium constituting the nonlinear waveguide 21B and then inject the control light and further pass a second pulse through the gain medium. In the above manner, a bit rate for the switch process is restricted in accordance with the time when the optical signal passes in the optical fiber loop 21. Additionally, as long as a conventional optical fiber loop is used in the NOLM optical switch, there is a limit of miniaturization of the apparatus.
In FIG. 3B, the Mach-Zehnder all-optical switch 30 has a structure wherein nonlinear waveguides 32A and 32B are provided in arms 31A and 31B constituting a Mach-Zehnder optical interferometer, respectively. A signal light injected from a waveguide 33A is divided into the arms 31A and 31B and is delivered to the nonlinear waveguides 32A and 32B. On the other hand, a signal light injected from a waveguide 33B is delivered to the nonlinear waveguides 32A and 32B at the different timing from each other. As a result, a differential phase shift arises between the signal lights passing through the arms 31A and 31B. Thus, when the signal lights are multiplexed, the differential phase shift causes the switching.
In the Mach-Zehnder all-optical switch 30, however, while the operation speed is not restricted due to the light passing time unlike the NOLM optical switch 20, it is necessary to place two arms in which nonlinear waveguides are incorporated in parallel and further provide parts serving to multiplex control lights in each of the arms. Thus, the Mach-Zehnder all-optical switch 30 cannot help having a large structure.
In FIG. 3C, the polarization-discriminating optical switch 40 delays one polarized component of an optical signal injected from a waveguide 41A by a birefringent crystal (polarization-discriminating delay circuit) 42 and simultaneously performs a phase shift for two polarized components of the optical signal in a nonlinear waveguide 43 on the identical optical axis. Furthermore, the resulting polarized components are restored in a polarization-discriminating delay circuit 44, and a polarizer 45 retrieves only a pulse whose phase is different based upon the polarized components. A control light from a waveguide 41B controls the nonlinear waveguide 43. In this system, it is possible to simplify the system structure because the system has only one arm of the optical interferometer unlike the Mach-Zehnder all-optical switch. However, since the optical signal in the optical fiber has a random polarized component, it is difficult to apply the polarization-discriminating optical switch 40 to an optical fiber communication system.
A description will now be given, with reference to FIG. 4, of a mode-locked laser used in the DEMUX processing system 10A in FIG. 2.
FIG. 4 shows a structure of a conventional mode-locked laser 50 formed of a laser diode.
In FIG. 4, gain areas 50A and 50B are provided in an optical cavity extending in the axis direction in the mode-locked laser 50. A saturable absorption area 50C is provided between the gain areas 50A and 50B. Electrodes 51A, 51B and 51C are provided for the gain areas 50A and 50B and the saturable absorption area 50C. A forward-biased actuating current is injected to the electrodes 51A and 51B, whereas a reverse-biased voltage is applied to the electrode 51C.
In FIG. 4, the mode-locked laser 50 has a structure in which the saturable absorption area 50C is placed at the center of the optical cavity with respect to the axis direction. According to this structure, a pulse light, which is called a colliding mode-locked pulse, is layered in the saturable absorption area 50C, thereby enhancing the effect of optical saturation. This saturable absorber periodically modulates the traveling light in the optical cavity, thereby generating an oscillation at a certain frequency in accordance with the length of the cavity. As mentioned above, this system performs the mode-locked operation.
FIG. 5 shows another structure of the mode-locked laser wherein those parts in FIG. 5 corresponding to the parts in FIG. 4 are designated by the same reference numerals and the description thereof will be omitted.
In FIG. 5, the laser diode in FIG. 4 is folded back at the center. A high reflective mirror 52 is provided on one end surface of the laser diode so as to shorten the cavity length in half. The laser diode in FIG. 5 is virtually equivalent to that in FIG. 4.
Apart from the above-mentioned mode-locked laser, a mode-locked laser in which an optical fiber is used is additionally presented.
As mentioned above, various types of optical switches and mode-locked lasers have been presented. However, the integration of these parts requires a sophisticated technique even if the parts are semiconductor elements. Thus, it is found extremely difficult to miniaturize the entire optical DEMUX system by using existing techniques.
A description will now be given, with reference to FIGS. 6A through 6C and FIG. 7, of conventional methods with respect to another technique according to the present invention, that is, a reshaping technique of an optical signal waveform without any conversion. This reshaping technique is quite difficult and any promising method is not presented even in conferences at present.
An optical signal deteriorates while the optical signal is traveling in an optical fiber. Three processes to recover the deteriorated optical signal are generally called “3R”, which originates from initial letters of the three processes. The 3R represents a reshaping process shown in FIG. 6A, a retiming process shown in FIG. 6B, and a regeneration process shown in FIG. 6C. The reshaping process modifies the intensity of a disordered optical signal so that “0/1” of the optical signal can be identified. The retiming process modifies the timing of an optical signal. The regeneration process recovers a weakened optical signal.
An optical signal waveform reshaping element 60 in which a wavelength converter is used to perform the 3R processes is presented in FIG. 7.
In FIG. 7, a wavelength converter 62 receives an input optical signal λs via an optical waveguide 61A together with a light having another wavelength λp from a continuous wave (CW) illuminant 61B. The wavelength converter 62, which is a light element generating saturable absorption, usually absorbs all lights from the CW illuminant 61B. However, if a signal light is injected from the waveguide 61A, absorption saturation is generated due to the signal light. As a result, the light from the CW illuminant 61B whose wavelength is λp is output without absorption. In this manner, the injected optical signal is supplied as an optical output of the wavelength converter 62.
When the wavelength converter 62 performs the absorption saturation process, it is possible to eliminate the disorder of the signal light intensity to some extent by the output of the CW illuminant 61B and the saturation characteristic of the wavelength converter 62. In this manner, the reshaping process in FIG. 6A is achieved. The output optical signal from the wavelength converter 62 is amplified when the output optical signal passes through an optical fiber amplifier 63. As a result, the retiming process in FIG. 6B is achieved.
A PLL circuit 64 receives an electrical signal corresponding to the fluctuation of a carrier density caused by the injected optical signal in the wavelength converter 62 and is synchronized with timing of the optical signal. The PLL circuit 64 actuates an electric field absorption (EA) modulator 65, and a continuous light from another continuous wave illuminant 66 is supplied to the EA modulator 65. Consequently, the EA modulator 65 modulates the continuous wave light from the continuous wave illuminant 66 synchronously with the timing of the injected optical signal.
It is possible to set the wavelength λs of the continuous light from the continuous wave illuminant 66, for example, to the same wavelength as the input signal light. As a result, the pulse light formed by the EA modulator 65 has an ideal waveform and timing for the input signal light.
A wavelength converter 67, which performs the saturable absorption, receives the optical signal amplified by the fiber amplifier 63 from the illuminant 61B and the optical signal modulated by the EA modulator from the illuminant 66. While the wavelength converter 67 is set to absorb the optical signal from the fiber amplifier 63 if there is no optical signal from the EA modulator 65, the wavelength converter 67 passes the light from the fiber amplifier 63 under the saturable absorption function if an optical signal is injected from the EA modulator 65.
As a result, the reshaping and retiming processes are performed for the optical signal produced by the fiber amplifier 63 according to the optical clock signal from the EA modulator 65, thereby achieving the reshaping in FIG. 6A and the retiming in FIG. 6B.
As mentioned above, it is possible to perform the 3R process for the signal light having the wavelength λs under the configuration shown in FIG. 7. However, the configuration is quite complicated as seen in the above description, and additionally the delicate control of the light intensity is required. Thus, the system is not sufficient for the practical use.